Immanuel Kant

Universal Natural History and Theory of Heaven

Part Two

Section Three

Concerning the Eccentricity of the Planetary Orbits and the Origin of Comets

We cannot make the comets a special class of celestial bodies entirely different from the family of planets. Here, as elsewhere, nature works by imperceptible stages. While it goes through all the series of changes, Nature links together distant qualities with ones close at hand, thanks to a chain of intermediate rungs. The eccentricity in the planets is the result of a lack of that impetus by which nature strives to make planetary movement precisely circular, something which, however, nature can never perfectly attain because of the intervening influence of various causes. However, the deviation from circular motion is greater at the larger distances from the sun than close by.

This condition goes through a constant scale with all possible levels of eccentricity from the planets right up to the comets. True, this interconnection seems to be severed in the case of Saturn because of a large gap which completely separates the family of comets from the planets. But in the first part we have remarked that there probably are still other planets beyond Saturn which are more like comets because of a greater deviation from circularity in their orbital path and that it is only through a lack of observations (or also the difficulty involved in such observations) that this affinity was not long ago revealed as clearly to eye as to the understanding.

In the first section of this part we have already referred to a cause which can render eccentric the orbit of a cosmic body developing out of the basic material suspended all around, even if we also assume that this body in all locations has forces moving it in a circular motion. Because the planet collects materials from location at a considerable distance from each other, where the orbital velocities are different, the materials collectively reach the planet with different degrees of inherent orbital velocity. These deviate from the velocity appropriate to the distance of the planet from the sun and thus induce an eccentricity for the planet insofar as these different impressed particulate motions fail fully to offset each other's deviation.

If the eccentricity was caused only by this, it would be moderate everywhere. Also it would be less significant with the small planets far from the sun than with the closer and larger planets, if we assumed that the particles of the basic material had previously had a true circular movement. Now, these estimates do not agree with observation, since, as has already been mentioned, the eccentricity increases with the distance from the sun, and the small size of the masses appears much rather to create an exception to an increase in eccentricity, as we see with Mars. Thus, we are forced to limit the hypothesis about the precise circular movement of the particulate basic materials, so that, while they very nearly attain the determined precision in the regions near the sun, they nevertheless admit wider deviations from that precision the further the elementary particles are suspended from the sun. Such an adjustment of the basic principle of the free circular movement of the basic material is more naturally appropriate. For regardless of the spatial diffusion, which seems to leave them free to limit each other at the point of completely balanced equilibrium of the central forces, no less considerable are the causes which hinder the attainment of this natural goal. The further the dispersed parts of the basic material are from the sun, the weaker the force which induces them to sink down. The resistance of the particles below, which should force them to bend their fall sideways and to take up a direction perpendicular to the radius of the circle, is diminished in proportion as these particles sink downward either to disappear into the sun or to assume an orbit in a region closer to the sun. The fact that this more distant material has a much lighter specific gravity does not permit it to acquire the downward movement, which is the basis for everything, with the force necessary to brush the resisting particles aside. Perhaps these distant particles still restrict each other in order finally to overcome this uniformity after a long time. Thus, among these distant particles already small masses have developed as the starting point of many celestial bodies, which, because they are assembled from weakly moving material, sink toward the sun with only an eccentric movement and on the way are increasingly diverted from a perpendicular fall by taking on more quickly moving pieces. Finally, however, they remain comets if the spaces in which they have developed have through the sinking down toward the sun or through the assembling in particular clusters, become cleansed and empty. This is the reason why the eccentricity of the planets and those celestial bodies called comets increases with the distance from the sun. Comets have their name for the very reason that in this characteristic they far exceed the planets. There are indeed two exceptions which violate the law concerning the increase in eccentricity with the increasing distance from the sun. We see them in the two smallest planets of our system, Mars and Mercury. But with the first the cause is presumably the vicinity of the large planet Jupiter, which through its power of attraction on its side of Mars deprives it of particles and thus only allows Mars a special area in the direction of the sun in which to extend itself. This brings with it an excessive central force and eccentricity. So far as Mercury, the lowest but also the most eccentric planet, is concerned, it is easy to believe that, because the sun's axial rotation does not yet by a long way equal Mercury's velocity, the resistance which the sun presents to the material in the space surrounding it not only deprives the nearest particles of their central movement but also this resistance could easily extend right out to Mercury, whose projectile velocity would on this account have been considerably diminished.

Eccentricity is the most significant mark of the comets. Their atmosphere and tail, which expand through the heat of their close approach to the sun, are only consequences of the eccentricity, although they have always served ignorance as uncommon images of horror, announcing to the common folk imaginary destinies. Astronomers, who pay more attention to the laws of motion than to the strangeness in the shape, notice a second characteristic distinguishing comets from planets, namely, unlike planets, comets do not confine themselves to the area of the zodiac, but establish their orbits in all celestial regions without restriction. This peculiarity has exactly the same cause as the eccentricity. The planets have confined their orbits to the narrow region of the zodiac because the elementary particles in the vicinity of the sun acquire circular movements which in each revolution have a tendency to intersect the interrelated plane and do not allow a body, once developed, to deviate from this plane towards which all the material from both sides presses. Thus, basic material from the spaces far from the mid-point which, because of the weak force of attraction, cannot attain free orbital movement for the very reason which produces eccentricity, is not capable of accumulating at this height on the plane interconnecting all planetary movement so as to maintain the bodies developed there on this particular path. Since it is not limited to a particular region, as is the case with the lower planets, the scattered basic material will far sooner develop on its own into celestial bodies equally easily on both sides by piling up far from the interconnecting plane or near to it. Therefore, comets will be fully free to descend toward us from all regions. However, those which first developed in a place not far above the planetary orbits will manifest less deviation from the limitations on their paths as well as less eccentricity. With the increasing distances from the mid point of the system, this lawless freedom of the comets in relation to their deviations increases and loses itself in the depths of the heavens in a total lack of orbital movement. This leaves the bodies developing in the outer regions free to fall toward the sun and establishes the last frontiers of the systematic arrangement.

In this outline of the comet's movements, I propose that, so far as their orbital direction is concerned, for the most part they have one in common with the planets. It seems to me that in the case of the comets close by this is undoubtedly the case. Also this similarity of form cannot get lost in the depths of the heavens before the point where the elementary basic stuff in the least energetic state of motion establishes the rotation which arises in all directions from the downward sinking. For the time required in the commonality of the movements lower down to align them in a common direction is, on account of the large distance, too long for them to be able to extend themselves that far, while the development of nature goes on in the lower region. There will also perhaps be comets which will establish their orbits in the direction opposite to this principle, namely, from east to west. However, I might equally almost persuade myself, for reasons which I am reluctant to cite here, that of the nineteen comets in which we have observed this peculiarity, in some of them an optical illusion may have given rise to this observation.

I must still note something about the masses of the comets and about the density of their material. For the reasons referred to in the previous section, according to the rules the development of these celestial bodies in the upper regions should proceed always according to the principle that, as the distance increases, their masses get larger. And we must also accept the fact that a few comets are larger than Saturn and Jupiter. But we do not have to hold that this quantity of the masses always increases in this manner. The scattering of the basic materials and the particular lightness of their particles make the development in the furthest region of cosmic space slow. The uncertain diffusion of this material in the entire infinite expanse of this space without any tendency to accumulate in the direction of any common plane initiates several small developments in place of a single considerable one. And the small centripetal force draws the largest portion of the particles down to the sun, without their having assembled themselves into masses.

The specific density of the stuff out of which the comets develop is more worthy of attention than the size of the masses. Presumably since they are developed in the uppermost reaches of the cosmic structure, the particles which compose them are of the lightest sort. We cannot doubt that this is the major cause of the vapour sphere and the tail, which distinguish them from the other celestial bodies. We cannot attribute this dispersal of the comet's material in a vapour to the effect of solar heat. A few comets in their approach to the sun hardly reach the depth of the Earth. Many remain between the orbits of Earth and Venus and then turn back. If such a moderate level of heat dissolves and thins out the material on the surface of the comets to this extent, then they would have to consist of the lightest material which undergoes, under the influence of heat, more thinning out than any material whatsoever in all nature.

Moreover, it is not possible to attribute the vapours which arise in such an accumulation from the comet to the heat which its body has left over from the earlier approaches to the sun. For indeed we must suppose that at the time of its development a comet has gone through several orbits with greater eccentricity and that these were reduced gradually. But the other planets, for which we could assume the very same, do not manifest this phenomenon. They would display it, if the varieties of the lightest material included in the composition of the planets had accumulated as much as they have with the comets.

The Earth has something in itself which we can compare with the dispersal of the comet's vapours and their tails [i.e., the Northern Lights]. The finest particles which the effect of the sun draws from Earth's outer surface pile up around one of the poles, when the sun directs the semi-circle of its orbit into the opposite hemisphere. The finest and most energetic particles arising in the hot equatorial regions, having attained a certain atmospheric altitude, are compelled by the effect of the sun's rays to move away and accumulate in those regions which at that period are directed away from the sun and buried in a long night. These particles compensate the inhabitants of the icy regions for the absence of the great light, which also at this distance sends them the effects of its heat. Just this same power of the sun's rays, which creates the Northern Lights, would bring out a vapour circle with a tail, if the finest and most elusive particles on the Earth were in exactly the same accumulations as would occur on the comets.


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